1. Field of the Invention
The present invention relates to an ultrasound diagnosis apparatus that transmits ultrasound waves to a subject and obtains an ultrasound image based on the waves reflected from the subject.
More specifically, the present invention relates to an ultrasound diagnosis apparatus that executes a correction process on a generated ultrasound image.
2. Description of the Related Art
An ultrasound diagnosis apparatus transmits, to a subject, ultrasound waves generated from transducers incorporated in an ultrasound probe. The ultrasound diagnosis apparatus receives, by the transducers, reflected waves from a subject tissue of the transmitted ultrasound waves, generates an ultrasound tomographic image of the subject from the received reflected waves, and displays the ultrasound tomographic image.
Signals received from a plurality of subject tissues adjacent to each other interfere with each other because having different phases.
This interference makes a vision different from when only amplitudes are composed, and generates a speckled image pattern. Such an image pattern is called speckle. This speckle prevents accurate observation of the position and shape of the boundary of the subject tissues.
The accurate observation is also prevented by occurrence of noise. Such speckle and noise occur not only in the spatial direction but also in the temporal direction.
Up to now, various processing methods for reducing speckle and noise have been proposed.
For example, a technique of using a temporal-direction IIR (Infinite Impulse Response) filter has been proposed. This technique is a technique of using a filter that, assuming an image acquired at time t is represented by I(t) and an image having been processed by the filter is represented by Jt, satisfies a relation of Jt=(1−a)It+aJt−1, wherein a is 1 or less. This technique enables reduction of speckle and noise that vary with time.
However, this technique reduces noise by adding the proportion of a of the past data Jt−1 to the proportion of (1−a) of the present data It.
Therefore, for a strenuously moving tissue such as the diaphragm and the heart, there is a problem that, in a superimposed image, each image of the strenuously moving tissue remains like an afterimage and blur or the like occurs.
In a case that an image is generated for each frame, which is the unit of a set of data necessary for generating one tomographic image, a motion vector of each image (image of an organ, for example) in a past frame is detected from the image.
Then, the position of an image in a next frame is predicted by using the motion vector, and a filter process or the like is performed at the predicted position.
With this technique, it is possible to perform image processing including displacement of an image with time.
However, as mentioned above, the process of detecting the motion vector is required in addition to the noise reduction process.
Therefore, there is a problem that the process gets slow.
Further, in order to define a tissue part, a speckle/noise reduction filter has been used conventionally.
As the speckle/noise reduction filter, a filter using a directional feature amount in a two-dimensional or three-dimensional space is used.
The directional feature amount is the size or direction of an edge. The edge is a part in which the luminance largely changes, such as the outside of a tissue. Also, the edge is a part other than noise.
As the abovementioned filter using the directional feature amount, it is possible to use a filter that blurs an image in the edge direction, or a filter that emphasizes the contrast of an image in a direction orthogonal to the edge direction. As the filter that blurs an image in the edge direction, it is possible to use, for example, a filter that obtains an average value of a row of points.
Further, as the filter that emphasizes the contrast in the direction orthogonal to the edge direction, it is possible to use, for example, a filter that, by a threshold, decreases the luminance outside the edge while increases the luminance inside the edge.
Use of the filter that blurs an image in the edge direction makes it possible to obtain an image with smooth shading in the edge direction.
Further, use of the filter that emphasizes the contrast of an image in the direction orthogonal to the edge direction makes it possible to obtain an image with an edge defined and a tissue part emphasized.
As a method for obtaining the abovementioned directional feature amount, a technique of using multiresolution decomposition is proposed.
Multiresolution decomposition is a method of decomposing arbitrary video signals into a plurality of videos having different frequency bands (spatial frequency bands).
As multiresolution decomposition, it is possible to use wavelet transform. This wavelet transform is decomposition into a video of low-frequency components and a video of high-frequency components that have a 2−1 size of an inputted video.
Based on videos obtained by the wavelet transform, the edge of an image of low-band spatial frequency is detected by using a diffusion filter, and the direction of the edge is obtained for each pixel.
A technique of smoothing pixels by using the filter that blurs an image in the tangent direction of the edge while sharpening pixels by using the filter that emphasizes the contrast in the normal direction of the edge is proposed (refer to, for example, Japanese Unexamined Patent Application Publication JP-A 2006-116307).
This enables definition of a tissue part in the video of low-frequency components.
In the image processing by multiresolution decomposition and the respective filters described in JP-A 2006-116307, a noise reduction process is performed with respect to the spatial direction.
However, in the technique described in JP-A 2006-116307, the image processing by multiresolution decomposition and the respective filters is not executed on an image including the temporal direction.
Therefore, such a method of performing the noise reduction process with respect to only the spatial direction may cause noise and speckle in an image with respect to the temporal direction.
Further, in a case that the filtering process with respect to the space is applied as it is, there is a problem that a time-phase delay occurs and an image immediately after generated (a real-time image) cannot be displayed.